I was also thinking about putting water in a plastic oven bag inside a (leaky) aluminum foil container or even just on top of a sheet to heat up over a fire - the aluminum would shield the plastic from direct flame. The question I have is although oven bags are designed to be used in microwaves would heating the bag cause chemicals to leach into the water?
Reynolds used to have aluminum foil cooking bags among their products. My sketchy memory thinks they were rated around 400°F and, necessarily, fairly heavy-duty foil But believe I've read they discontinued the product line (and couldn't find any sources online). Probably competing products out there. If plastic bag could be made workable, as you suggest, then foil cooking bag should do even better.